Add base direction as a fourth element of literals.

spec/index.html

@@ -73,7 +73,10 @@
   </ul>
 
   <p>RDF 1.2 introduces <a>quoted triples</a> as another kind of <a>RDF term</a>
-    which can be used as the <a>subject</a> or <a>object</a> of another <a>triple</a>.</p>
+    which can be used as the <a>subject</a> or <a>object</a> of another <a>triple</a>.
+    RDF 1.2 also introduces <a>directional language-tagged strings</a>
+    containing a <a>base direction</a> element allowing the
+    initial text direction to be specified when presented in a user agent.</p>
 
   <p>RDF 1.2 Concepts introduces key concepts and terminology for RDF 1.2, discusses
     datatyping, and the handling of <a>fragment identifiers</a> in IRIs within
@@ -153,7 +156,9 @@ <h3>Resources and Statements</h3>
     <a>datatypes</a> that define the range of possible
     values, such as strings, numbers, and dates. Special kind of literals,
     <a>language-tagged strings</a>, denote
-    plain-text strings in a natural language.</p>
+    plain-text strings in a natural language, and
+    <a>directional language-tagged strings</a>, denote
+    plain-text strings in a natural language and include an initial text direction.</p>
 
     <p>Asserting an <a>RDF triple</a> says that <em>some relationship,
     indicated by the <a>predicate</a>, holds between the
@@ -669,8 +674,8 @@ <h2>Literals</h2>
 
     <p>Literals are used for values such as strings, numbers, and dates.</p>
 
-    <p>A <dfn data-local-lt="RDF literal">literal</dfn> in an <a>RDF graph</a> consists of two or three
-      elements:</p>
+    <p>A <dfn data-local-lt="RDF literal">literal</dfn> in an <a>RDF graph</a> consists of
+      two, three, or four elements:</p>
 
     <ul>
       <li>a <dfn>lexical form</dfn>, being a Unicode [[!UNICODE]] string,
@@ -684,32 +689,48 @@ <h2>Literals</h2>
         language tag MUST be well-formed according to
         <a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
         of [[!BCP47]].</li>
+      <li>if and only if the <a>datatype IRI</a> is
+        <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
+        a non-empty <a>language tag</a>
+        that MUST be well-formed according to <a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
+        of [[!BCP47]].
+        and a <dfn>base direction</dfn> that MUST be either `ltr` or `rtl`.</li>
     </ul>
 
     <p>A literal is a <dfn>language-tagged string</dfn> if the third element
-      is present. Lexical representations of language tags MAY be converted
-      to lower case. The value space of language tags is always in lower
-      case.</p>
+      is present and the fourth element is not present.
+      Lexical representations of language tags MAY be converted
+      to lower case.
+      The value of language tags is always treated as being in lower case.</p>
+
+    <p>A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
+      if both the third element and fourth elements are present.
+      Language tags are treated identically to <a>language-tagged string</a>,
+      and the fourth element , <a>base direction</a> MUST BE either `ltr` or `rtl` in lower case.</p>
 
     <p>Please note that concrete syntaxes MAY support
       <dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
-      <a>lexical form</a> without any <a>datatype IRI</a> or <a>language tag</a>.
+      <a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
       Simple literals are syntactic sugar for abstract syntax
       <a>literals</a>
       with the <a>datatype IRI</a>
       <code>http://www.w3.org/2001/XMLSchema#string</code>
       (which is commonly abbreviated as <code>xsd:string</code>).
       Similarly, most concrete syntaxes represent
-      <a>language-tagged strings</a> without
-      the <a>datatype IRI</a> because it always equals
-      <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>).</p>
+      <a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
+      the <a>datatype IRI</a> because it always equals either
+      <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
+      or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.</p>
 
     <p>The <dfn>literal value</dfn> associated with a <a>literal</a> is:</p>
 
     <ol>
       <li>If the literal is a <a>language-tagged string</a>,
         then the literal value is a pair consisting of its <a>lexical form</a>
         and its <a>language tag</a>, in that order.</li>
+      <li>if the literal is a <a>directional language-tagged string</a>, then the literal value is
+        a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
+        likewise in that order.</li>
 
       <li>If the literal's <a>datatype IRI</a> is in the set of
         <a>recognized datatype IRIs</a>, let <var>d</var> be the
@@ -732,12 +753,16 @@ <h2>Literals</h2>
         not defined by this specification.</li>
     </ol>
 
-    <p><dfn data-local-lt="term-equal">Literal term equality</dfn>: Two literals are term-equal (the same
-      RDF literal) if and only if the two <a>lexical forms</a>,
-      the two <a>datatype IRIs</a>, and the two
-      <a>language tags</a> (if any) compare equal,
-      character by character. Thus, two literals can have the same value
-      without being the same RDF term. For example:</p>
+    <p><dfn data-local-lt="term-equal">Literal term equality</dfn>:
+      Two literals are term-equal (the same <a>RDF literal</a>)
+      if and only if the two <a>lexical forms</a>,
+      the two <a>datatype IRIs</a>,
+      the two <a>language tags</a> (if any), and
+      the two <a>base directions</a> (if any),
+      all compare equal, character by character.
+      Thus, two literals can have the same value
+      without being the same <a>RDF term</a>.
+      For example:</p>
 
     <pre>
       `"1"^^xs:integer`
@@ -747,7 +772,32 @@ <h2>Literals</h2>
     <p>denote the same <a data-lt="literal value">value</a>, but are not the
       same literal <a>RDF terms</a> and are not
       <a>term-equal</a> because their
-      <a>lexical form</a> differs.</p>
+      <a>lexical forms</a> differ.</p>
+
+    <section id="section-text-direction" class="informative">
+      <h3>Initial Text Direction</h3>
+
+      <p>The <a>base direction</a> of a <a>directional language-tagged string</a>
+        provides a means of establishing the initial direction of text,
+        including text which is mixture of right-to-left and left-to-right scripts.
+        The [[[?UAX9]]] [[?UAX9]] provides support for automatically rendering
+        a sequence of characters in logical order,
+        so that they are visually ordered as expected.
+        However, this is not sufficient to correctly render bidirectional text.</p>
+
+      <p>For example, the text `"פעילות הבינאום, W3C"` with language tag "he" (for Hebrew) may be displayed
+        incorrectly unless the <a>base direction</a> of `rtl` (for right-to-left)
+        is also provided.
+        When this is provided to a user agent,
+        for example using the HTML `dir` attribute,
+        it would be correctly presented as follows: <div lang="he" dir="rtl">פעילות הבינאום, W3C</div></p>
+
+      <p class="note">The absence of a <a>base direction</a> does not necessarily imply that
+        the text has no initial text direction;
+        as described in [[[?UAX9]]],
+        strings may be embedded within structures which establish an <em>embedding direction</em>,
+        which determines the default bidirectional orientation of text.</p>
+    </section>
   </section>
 
   <section id="section-blank-nodes">
@@ -1483,17 +1533,14 @@ <h2>Security Considerations</h2>
 
 <section id="internationalization" class="appendix informative">
   <h2>Internationalization Considerations</h2>
-  <p>RDF is restricted to representing string values with left-to-right or right-to-left direction indicators.
-    RDF provides a mechanism for specifying the language associated with
-    a string (<a>language-tagged string</a>),
-    but does not provide a means of indicating the base direction of the string.</p>
-
   <p>Unicode provides a mechanism for signaling direction within a string
-    (see [[[UAX9]]] [[UAX9]]),
-    however, when a string has an overall base direction which cannot be determined by the
+    (see [[[UAX9]]] [[UAX9]]).
+    RDF provides a mechanism for specifying the <a>base direction</a>
+    of a <a>directional language-tagged string</a>
+    to signal the initial text direction of a string.
+    When a string has an overall base direction which cannot be determined by the
     beginning of the string, an external indicator is required,
-    such as the [[HTML]] <a data-cite="HTML/dom.html#the-dir-attribute">dir attribute</a>,
-    which currently has no counterpart for <a>RDF literals</a>.</p>
+    such as the [[HTML]] <a data-cite="HTML/dom.html#the-dir-attribute">dir attribute</a>.</p>
 
   <p>[[[JSON-LD11]]] [[JSON-LD11]] introduced the
     <a data-cite="JSON-LD11#the-i18n-namespace">i18n namespace</a> to use
@@ -1636,6 +1683,9 @@ <h2>Changes between RDF 1.1 and RDF 1.2</h2>
       for informative definition of a <a>quad</a>.</li>
     <li>Added <a href="#section-quoted-triples" class="sectionRef"></a>
       and definitions for <a>quoted triple</a> and <a>asserted triple</a>.</li>
+    <li>Added the <a>base direction</a> element as part of 
+      a <a>literal</a>,
+      and a description of its use in <a href="#section-text-direction" class="sectionRef"></a>.</li>
   </ul>
 
   <p class="note">A detailed overview of the differences between RDF versions&nbsp;1.0